The present invention relates to a glass suitable for manufacturing high temperature lamp envelopes such as tungsten-halogen lamp envelopes, as well as glass suitable for manufacturing high temperature glass filters and glass filter lenses, such as those used with tungsten-halogen lamps. More specifically, the present invention relates to Nd2O3 containing glass compositions that provide tungsten-halogen lamps with a higher correlated color temperature (CCT), thereby making the emitted light whiter, which can aid illuminated surroundings contrast.
In the automotive industry there is a continued need for automobile headlights which provide better illumination of the surrounding environment. In recent years tungsten-halogen lamps have gained popularity over conventional incandescent sealed beam lamps due to several advantages which include: a whiter light emitted; a smaller size lamp produces an equivalent or even greater quantity of light; the intensity of the illumination remains virtually constant over the life of the lamp; and, the service life is significantly longer. In spite of these advantages, however, tungsten-halogen lamps can be improved especially with respect to illumination and contrast performance. This may be achieved, for example, by manufacturing the lamp envelopes out of glass that gives the tungsten-halogen lamp a higher CCT.
Because tungsten-halogen lamps operate at high temperatures, generally between about 500-700xc2x0 C., suitable glasses for these lamp envelopes must be thermally stable (resist devitrification) and withstand thermal deformation at high temperatures. Additionally, the glass must permit a seal to the molybdenum leads while maintaining integrity through lamp operation. The glass must also be able to be economically formed into good quality tubing or pressed into filters and lens filter shapes.
Hence, much research has been concentrated upon alkaline earth aluminosilicate glasses because such are suitable for mass production of tungsten-halogen lamp envelopes, while concurrently manifesting the properties necessary for lamp operation.
U.S. Pat. No. 3,496,401 (Dumbaugh) describes the basic mechanism underlying the operation of tungsten-halogen incandescent lamps with specific reference to tungsten-iodide lamps. U.S. Pat. No. 3,496,401 discloses alkaline earth metal aluminosilicate glass compositions suitable as envelopes for such lamps, wherein the glass compositions consist essentially, expressed in weight percent on the oxide basis, of 10-25% alkaline earth metal oxide, 13-25% Al2O3, 55-70% SiO2, 0-10% B2O3, and less than 0.1% alkali metal oxide.
U.S. Pat. No. 3,978,362 (Dumbaugh et al.) discloses glasses designed for tungsten-bromine lamp envelopes displaying strain points greater than 700xc2x0 C., liquidus viscosities of at least 100,000 poises, operable melting temperature no higher than 1550xc2x0 C., liquidus temperature less than 1200xc2x0 C., and coefficients of thermal expansion between 48-55xc3x9710xe2x88x927/xc2x0 C., the glass composition consisting essentially, in weight percent, of 14-21% CaO, 0-5% MgO, 0-7% BaO, the total CaO+MgO+BaO being at least 19%, 13-16% Al2O3, 0-10% SrO and/or La2O3 and 58-63% SiO2.
U.S. Pat. No. 4,060,423 (Thomas) described another group of glass compositions particularly designed for use envelopes for tungsten-halogen lamps. Those glasses are characterized as exhibiting a liquidus temperature no greater than 1250xc2x0 C., a strain point of at least 725xc2x0 C., and a coefficient of expansion of about 42-48xc3x9710xe2x88x927/xc2x0 C. The compositions therefor consist essentially, expressed in terms of weight percent on the oxide basis, of 55-68% SiO2, 15-18% Al2O3, 6-13% CaO, and 6-16% BaO, wherein the weight ratio Al2O3:CaO+BaO is about 0.6:1 to 1:1.
U. Pat. No. 4,255,198 (Danielson et al.) discloses glasses suitable for sealing to molybdenum metal, in tungsten-halogen lamps, and having a strain point in excess of 730xc2x0 C., a liquidus temperature of at least 40,000 poises, axial compression at room temperature not exceeding 350 PPM and axial compression or tension not exceeding 150 PPM at 500xc2x0 C., a coefficient of thermal expansion between 43-48xc3x9710xe2x88x927/xc2x0 C. The glasses have a composition consisting essentially, in terms of weight percent on the oxide basis, of: 62-64% SiO2, 14-16% Al2O3, 10-13% CaO, and 7-9% SrO.
U.S. Pat. No. 4,302,250 (Danielson) discloses glasses for use as envelopes for tungsten-halogen lamps displaying strain points higher than 750xc2x0 C., liquidus viscosities of at least 40,000 poises, liquidus temperatures below 1300xc2x0 C. and coefficients of thermal expansion between 48-55xc3x9710xe2x88x927/xc2x0 C., consisting essentially, in weight percent, of 11-14% CaO, 2-6.5% SrO+BaO, consisting essentially of 0-4% SrO and 0-5% BaO, 16.5-18.5% Al2O3, and 64-68% SiO2.
U.S. Pat. No. 4,605,632 (Elmer) discloses a high silica glass for the production of envelopes for tungsten-halogen lamps, consisting essentially, by weight, of 1-2.5% Al2O3, 0.25-1% CaO, 0.1-0.25 Na2O and/or K2O, 2.5-3.5% B2O3, 0.15-0.3% F., and the remainder SiO2.
U.S. Pat. No. 4,394,453 (Dumbaugh) describes a glass composition suitable for tungsten-halogen lamps, consisting essentially, expressed in weight percent on the oxide basis, of 60xc2x11.5% SiO2, 17.0xc2x11% Al2O3, 5.0xc2x10.8% B2O3, 11.4xc2x10.8% CaO, and 7.5xc2x10.8% MgO. The physical properties of the disclosed glasses are a strain point of at least 670xc2x0 C., a coefficient of thermal expansion of 42-45xc3x9710xe2x88x927/xc2x0 C., a liquidus temperature below 1100xc2x0 C., and a liquidus viscosity greater than 20,000 poises.
U.S. Pat. No. 4,409,337 (Dumbaugh) discloses glasses for tungsten-halogen lamps which exhibit a strain point in excess of 665xc2x0 C., a liquidus temperature no higher than 1125xc2x0 C., a liquidus viscosity of at least 50,000 poises, and a coefficient of thermal expansion between about 42-46xc3x9710xe2x88x927/xc2x0 C., the composition consisting essentially, in terms of weight percent on the oxide basis, of 56-59% SiO2, 16-17% Al2O3, 4.5-5.25% B2O3, 7.5-9.25% CaO, 5.5-6.25% MgO, and 5-9% MgO, wherein the sum of SiO2+Al2O3, will not exceed about 75%, the weight ratio SiO2:Al2O3 is maintained between about 3.1-3.7 and the weight ratio CaO:MgO is held between about 1.2-2.0.
U.S. Pat. No. 4,693,987 (Danielson) discloses a glass for use as envelopes for tungsten-halogen lamps especially where molybdenum metal wires are used, the glass having strain points between 718xc2x0-725xc2x0 C., liquidus viscosities of at least 50,000 poises, liquidus temperatures below 1145-1180xc2x0 C. and coefficients of thermal expansion between 46.2-50.1xc3x9710xe2x88x927/xc2x0 C., consisting essentially, in weight percent, of 10.7-11.9% CaO, 9.1-13.1% BaO, 14.6-15.1% Al2O3, and 60.5-62.4% SiO2.
Correlated Color Temperature (CCT), also referred to herein as lamp color temperature, is used in the lighting industry to describe and compare the color appearance of lamps. CCT refers to the temperature of a blackbody radiator whose perceived color most closely resembles that of a given light source. As the temperature of a blackbody is raised, its color changes from a dull red to bright red, then orange, yellow, white and finally blue. Thus, lower temperatures imply longer wavelengths and xe2x80x9cwarmerxe2x80x9d colors; higher temperatures imply shorter wavelengths and xe2x80x9ccoolerxe2x80x9d colors. In particular, typical automotive tungsten-halogen lamps are warm in color and show CCT ranging from 2800-3200K; a value given by the fact that tungsten-halogen lamps produce much of their light output in the yellow to red wavelength region of the spectrum between 560-700 nm and less in the blue to green wavelength region of the spectrum between 400-560 nm. One method of raising the CCT in tungsten-halogen lamps is to selectively absorb a small percentage of visible radiation in the longer wavelength region (yellow to red) while fully transmitting in the shorter wavelength region (blue to green). In terms of color temperature, absorbing or xe2x80x9csubtractingxe2x80x9d these higher wavelengths of energy results in a shift in CCT to higher temperatures. A method of absorbing the longer wavelengths is to dope the glass envelope with specific cations in the form of elemental oxide material, in particular neodymium oxide.
Neodymium containing glass is known. It has historically found employment in the aviation and navigation fields. Long recognized as a glass coloring agent, neodymium, a rare-earth element, possesses an absorption spectra that extends over both the visible and invisible regions, transferring practically unchanged to glasses. It is also known that neodymium""s major absorption of light is in the yellow region of the visible spectrum, between 568-590 nm. Illuminated objects seen through neodymium containing glass look very clear in the surrounding environment because the red and green tones are accentuated due to the yellow absorption.
Recently, neodymium containing glass has been disclosed in U.S. Pat. No. 5,548,491 (Karpen) for the production of motor vehicle headlights to reduce the visual discomfort from oncoming cars at night. Karpen discloses neodymium doped soda-lime silica glass for incandescent lamps, and neodymium doped borosilicate or quartz glass for tungsten-halogen lamps. Although neodymium oxide in the range of 5-30% by weight is disclosed, there are no specific glass compositions provided.
U.S. Pat. No. 4,315,186 (Hirano et al.) discloses a reflective electric lamp with a neodymium doped front lens section fused to a reflective mirror section. The front lens section only is formed from a neodymium containing glass, Nd2O3 accounting for 0.5-5% by weight. Again no specific glass compositions are disclosed. Borosilicate glass is given as an example of a glass material suitable for the manufacturing of the front lens mirror.
However, what the prior art has failed to do and what this invention provides is a range of compositions within the alkaline earth aluminosilicate system which in combination with Nd2O3 enables optimum tungsten-halogen lamp construction, thereby providing the necessary physical properties for successful and optimum lamp operation, while increasing the CCT above 3200K, preferably above 3400K.
The present invention resides in neodymium containing glass compositions for tungsten halogen lamp envelopes, as well as for glass filters and glass filter lenses for use with tungsten halogen lamps, the compositions consisting essentially in terms of weight percent on the oxide basis of about 50-62% SiO2, 10-17% Al2O3, 0-6% B2O3, 3.6-10% CaO, 0-7.5% MgO, 0.1-0.3% SrO, 2.4-18% BaO, 0-1% ZnO, 1-8% Nd2O3. Accordingly, it is a principal object of this range of glass compositions to exhibit the following physical properties: strain point between about 665xc2x0 C. to about 750xc2x0 C., coefficient of thermal expansion from 25xc2x0 C. to the glass set point, 5xc2x0 C. higher than the strain point, between about 49-59xc3x9710xe2x88x927/xc2x0 C., liquidus temperature below about 1320xc2x0 C., viscosity at liquidus temperature greater than about 2,000 poises up to about 170,000 poises, and transmission at 585 nm from about 5 to 60% for a 1.2 mm thick piece of glass.
In accordance with the present invention, the preferred glass compositions consists essentially, expressed in terms of weight percent on the oxide basis, of about 53-57% SiO2, 13-16.5% Al2O3, 0-5.8% B2O3, 5.3-7.7% CaO, 0-7.4% MgO, 0.1-0.3% SrO, 7.7-18% BaO, 0-0.7% ZnO, 1.5-4.5% Nd2O3. Accordingly, it is a principal object of the preferred range of glass compositions to exhibit the following physical properties: strain point between about 665xc2x0 C. to about 730xc2x0 C., coefficient of thermal expansion between about 49-59xc3x9710xe2x88x927/xc2x0 C., liquidus temperature below about 1250xc2x0 C., viscosity at liquidus temperature greater than about 30,000 poises, and transmission at 585 nm from about 18-60% for a 1.2 mm thick piece of glass.
Another object of the instant invention pertains to an increase in the lamp color temperature, exhibited by a tungsten halogen lamp containing a glass envelope, a glass filter lens, or a glass filter prepared from the preferred glass compositions, from about 3 to 7% above a similar glass with no Nd2O3.
An additional object of the instant invention pertains to the total visible lumen or candle power loss, exhibited by a tungsten halogen lamp containing a glass envelope, a glass filter lens or a glass filter prepared from the preferred glass compositions, limited to less than about 10%.
Additional features and advantages of the invention will be set forth in the detailed description that follows, and in part, will be readily apparent or recognized to those skilled in the art by practicing the invention as described herein.
It is to be understood that both the foregoing general discussion and the following detailed description and examples provided herein are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed.